Class inspection: show known subclasses/implementors

Similarly to what the official Javadoc tool does, the Inspector:

* when inspecting a given class, could render its known subclasses.
* when inspecting a given interface, could render its known subinterfaces and implementing classes.

Start by locating the Inspector implementation and the existing rendering path for inspected classes and interfaces. Compare its output with the behavior described for Javadoc. Done means a class displays known subclasses, while an interface displays known subinterfaces and implementing classes.

Written by the indexing model from the issue text.